How to render rotating propeller?
I think you mean the motion blur effect.

In Cycles render engine, you can first animate the propeller with a very fast rotation, and toggle Motion Blur in Render tab, then set a appropriate Shutter time:



For Blender Internal render engine, there is also a Sampled Motion Blur panel in the Render tab: 



For compositing, you can enable Vector pass in Render Layers tab, and use Vector Blur node to make use of it. Example: